This invention generally relates to improvements in the upper internals assembly of a nuclear reactor, and is specifically concerned with a combined support column and guide tube which serves both to support the upper core plate of the reactor, over its fuel rod assemblies, as well as to receive and guide control rod assemblies into the fuel rod assemblies.
The core of a modern nuclear reactor of the type used to generate electrical power generally includes an upper internals assembly disposed over a lower core barrel. The lower core barrel houses an array of fuel rod assemblies which generate heat as a result of a controlled fission reaction that occurs in the uranium oxide pellets present in their individual fuel rods. Water is constantly circulated from the lower core barrel through the upper internals and out through outlet nozzles provided in the walls of an upper core barrel in order to transfer the heat generated by the fuel rod assemblies to heat exchangers which ultimately convert this heat into usable, nonradioactive steam.
The rate of the fission reaction taking place within the fuel rod assemblies is regulated by means of a control rod assembly. Each of these control rod assemblies is formed from an array of stainless steel tubes containing a neutron absorbing substance, such as silver, indium or cadmium. These stainless steel tubes (known as "rodlets" in the art) are suspended from a spider-like bracket. A reciprocable drive rod is connected to the spider like bracket for either inserting or withdrawing the rodlets of the control rod assembly deeper into or farther out of each of the fuel rod assemblies in order to modulate the amount of heat generated thereby.
The upper internals assembly includes an upper core barrel arranged in tandem with the lower core barrel of the reactor. The ceiling of the upper core barrel is formed from an upper support plate. The peripheral edge of this support plate is seated around the upper edge of the upper core barrel. Both the support plate and the upper core plate which underlies it include a plurality of apertures for both conducting the stream of hot, pressurized water exiting the fuel rod assemblies to the heat exchangers, as well as for conducting a control rod assembly either into or out of each of the fuel rod assemblies. Separate guide tubes are provided between apertures in both the support and ore barrel plates which are aligned with each other and with one of the fuel assemblies in the lower core barrel. The purpose of these guide tubes was to align and guide the relatively long and flexible rodlets of the control rod assemblies into a particular fuel assembly. Additionally, separate support columns are connected between the upper support plate and the underlying upper core plate in order to suspend and support the upper core plate over the fuel rod assemblies contained within the lower core barrel.
To facilitate the servicing of the components within the upper core barrel of such prior art nuclear reactors, the guide tubes had to be removable. Accordingly, each of the guide tubes included a pair of opposing, apertured lugs at its bottom end for receiving the top ends of a pair of opposing guide pins which were mounted on either side of one of the control rod-receiving apertures in the upper core plate. A mounting flange was provided at the upper end of each of the guide tubes. This flange sat upon the upper surface of the support plate when the guide tube was completely slid through a support plate aperture, thereby suspending the guide tube over its respective aperture in the upper core plate so that the guide pins in the upper core late provided only lateral support for the guide tube. To minimize corrosion problems, the guide pins in such designs were formed from Inconel 750, while the support plate, upper core plate and other components of the guide tubes were formed from No. 304 stainless steel.
The Applicants have observed that a number of problems have resulted from such prior art upper internals designs. For example, the differences in the coefficient of thermal expansion between the Inconel which forms the guide tubes and the stainless steel which forms the support plate and the upper core plate have generated stresses in the guide pins that have occasionally caused these pins to break due to stress corrosion cracking. Such pin breakage has had the immediate negative consequence of introducing large pieces of metallic debris into the pressurized stream of water flowing up through the upper core plate. Such debris can, of course, damage the primary system circulation pumps. Additionally, after the loss of one or more of the guide pins, the currents generated by this rapidly flowing stream of water (which typically moves at a rate of 20 feet per second) can cause the guide tube to wobble out of alignment with its respective aperture and possibly strike and damage adjacent guide tubes. Still another problem associated with such prior art guide tubes stems from the fact that they are not designed to bear any significant tensile load. Hence, separate support columns must be provided for suspending the upper core plate over the fuel rod assemblies. The use of two separate types of columns within the upper core barrel complicates both the assembly and disassembly of this particular region of the reactor, which in turn greatly increases the time required for routine maintenance in the upper internals of the reactor. Finally, while such prior art guide tubes typically include an array of ports in their walls for dissipating the pressure generated therein by the water flowing upwardly from the lower core barrel, the Applicants have observed that the port sizes and configurations have not been entirely successful in preventing localized jets of water from forming in the interior of these tubes. Such localized jets have been found to induce vibrations in the long and slender rodlets of the control rod assemblies, which in turn can cause them to rub and fret against the components surrounding them.